Distortion compensation can be categorized as being either postdistortion or predistortion compensation or a combination of both. Postdistortion compensation is compensation that is provided after distortion is introduced into an information signal while predistortion compensation is compensation for distortion prior to the introduction of the distortion.
In many communications applications, the application of known distortion compensation techniques results in signal noise enhancement. This results from the fact that the information signal is subjected to more distortion than the noise. One example of this is in dial-up modem applications, where the communications channel includes two-wire subscriber loops, each having an associated amplitude versus frequency distortion characteristic, and a four-wire transmission system therebetween. Most of the signal noise is introduced in the four-wire transmission system. Accordingly, any information signal propagating through the communications channel is subjected to amplitude distortion within both subscriber loops while the noise is only subjected to amplitude distortion within one subscriber loop. The process of using linear equalizers to compensate for the distortion to which the information signal has been subjected results in overequalization or enhancement of the noise signal.
The problem of noise enhancement is not severe for typical voice and low-speed data applications. However, as the data rate increases, such noise enhancement becomes intolerable as it substantially interferes with attainment of required bit error rates. Accordingly, compensation for such noise enhancement is required.
One prior an technique for addressing noise enhancement involves the use of decision feedback equalizers in each receiver to "whiten", i.e., provide a substantially fiat amplitude-versus-frequency spectrum for the noise at the equalizer output. The problem with this technique is that a decision feedback equalizer is an inherently complex device to implement and is subject to error propagation. Another technique, known as the "Tomlinson" technique, is to adapt a decision feedback equalizer using a priori known data, thereby eliminating the problem of error propagation, and then transmitting the coefficients of this equalizer to a remote transmitter wherein such coefficients are used to predistort the transmitted signal. The shortcoming of this technique is that it requires the availability of an error-free communications channel for coefficient transmission. Such a channel is not always available when required.
More recently, another technique, disclosed in patent application Ser. No. 07/630,636, filed Dec. 20, 1990 now U.S. Pat. No. 5,251,328 issued to D. G. Shaw on Oct. 5, 1993 and assigned to the present assignee, addresses the problem of noise enhancement and error propagation by predistorting the transmitted information to compensate for less than all of the distortion introduced in the communications channel. This technique involves the transmission of a signal having a frequency spectrum at least as wide as that of the information signal to be transmitted through the communications channel during a predesignated time period and the analysis of the error signal from an adaptive filter for determining the appropriate amount of predistortion. While this arrangement provides satisfactory performance, the need for an adaptive filter and associated circuitry for analyzing the error signal may increase implementation costs beyond the desired cost objectives. In addition, an allocation of the required predesignated time period may not be available.
Accordingly, it would be desirable if a distortion compensation technique could be devised which addresses the problem of noise enhancement and does not have the limitations associated with the prior art.